"A retrograde dopant profile" refers to a dopant profile which has a lower dopant concentration at a surface of an object than in the body of the object. Retrograde dopant profiles are known to be useful in the channel region of a metal-oxide-silicon field effect transistor ("MOSFET") to increase mobility and drive current in the channel region. However, there is an increasing need, as feature sizes of circuits become continuously smaller, to increase mobility and drive current in the channel region, particularly in short channel devices.
Since the advent of semiconductor devices, the minimum feature size has continuously decreased. In order to further decrease the minimum feature size, which is already well into the sub-micron realm, it is important to not only reduce the horizontal dimensions of the devices, but to also reduce the vertical dimensions such as, for example, the retrograde dopant profiles described above.
Conventionally, retrograde dopant profiles may be formed by implanting dopant ions into a semiconductor wafer so that the peak concentration is below the surface of the wafer. However, there is a need to form retrograde dopant profiles which are shallower and more abrupt in order to accommodate the continuously decreasing depths.
Accordingly, there is a need for a method of forming a more abrupt and shallow retrograde dopant profile than conventionally known.